The conventional wisdom related to starting fuel cells, particularly for automotive applications, when they have been subjected to freezing temperatures, is to heat the fuel cell prior to starting it, which means a significant delay before the vehicle can be operated.
As is described in U.S. patent application Ser. No. 10/390,439 filed Mar. 17, 2003, now U.S. Pat. No. 6,673,481, water in the porous water transport plates can reach or exceed freezing temperatures without doing damage to the water transport plates, primarily because of the small size of the pores therein. As is also disclosed in that application, the water that remains in the pores of the water transport plates aids in extending the period of time that a fuel cell can be operated during a startup at subfreezing temperatures before there is need to provide coolant to the fuel cell, or to operate the water management system of the fuel cell.
In PEM fuel cells which are designed for use in automotive applications, the use of insulation has heretofore not been a practical solution to the freezing of the fuel cells, nor to the starting of the fuel cells in a frozen condition when the fuel cells are subjected to subfreezing temperatures. This is because the volume of the fuel cell plus the volume of the insulation exceeds the volume which can be allocated for the power plant in an automotive application.
Other conventional wisdom has been to reduce the amount of water which remains in the power plant, thus to reduce the amount of water which has to be thawed and heated prior to operating of the fuel cell power plant.
PEM fuel cells that contain porous water transport plates can operate at high current densities at a cell temperature of 0° C. if the water in the water transport plate is not totally frozen. This is because the pressure of the oxidant air is sufficient to force product water through the porous water transport plate into the water channels associated with the water transport plate; and is sufficient to prevent flooding of the hydrophilic cathode support plate.
On the other hand, if the water in the porous water transport plates is totally frozen, i.e., a hard freeze, or if the fuel cell utilizes bi-polar separator plates that are solid, the product water is not easily removed, and low to moderate current densities must be maintained until the cells reach operating temperatures.